Wideband Sub-6 GHz MIMO Antenna for Full-Screen Metal Rim Smartphones

A wideband 8-element multiple-input multiple-output (MIMO) antenna for fifth-generation (5G) smartphones with a full-screen and a metal rim is presented. A single antenna element consists of two slots, open and closed, both placed solely on the metal rim. Wideband operation is based on the interactions of multiple slot modes, their simultaneous excitation, and overlapping resonance bands. The antenna occupies only the metal rim and does not have any matching circuits or decoupling structures. This simple zero ground clearance design enables a full screen which is a highly desirable property in modern mobile devices. A prototype of the proposed antenna design is manufactured and the measurements verify that the design covers a wide frequency band of 3.4–6.1GHz, isolation over 11.7dB, and good total efficiency of 57.6–84.9%. The performance of the antenna with the user’s hand is verified with simulations.


I. INTRODUCTION
The limited frequency spectrum in sub-6 GHz bands requires high spectral efficiency in fifth-generation (5G) communication.To achieve good channel capacity, high-order multiple-input multiple-output (MIMO) antennas are utilized in smartphones.When several frequency bands should be covered, the number of antennas easily becomes very large and the benefit of MIMO technology may be lost if high coupling of the antenna elements inside a small device deteriorates the data transfer capacity.Therefore, the design of efficient MIMO antennas for smartphones has become a popular research topic.
Recently, numerous compact high order MIMO antenna designs have been proposed in the sub-6 GHz bands.Singleband antennas [1], [2], [3], [4], [5], [6], [7], as well as dual-band antennas [8], [9], [10], [11], [12], [13], [14], are typically based on single resonances and they tend to cover only a narrow frequency band.The required small size of a The associate editor coordinating the review of this manuscript and approving it for publication was Bilal Khawaja .single antenna element limits the impedance bandwidth and, e.g., the Wireless Local Area Network (WLAN) 5 GHz band (5170-5835 MHz) might be missing.When new frequency bands are allocated to 5G, these solutions are not adequate since the total number of antennas in a smartphone becomes very high if all sub-6 GHz bands require a separate antenna.
The number of separate antennas can be reduced by using wideband designs.The challenge in these designs, however, is how to handle coupling between the antenna elements in a wide frequency band.For single narrow-band antennas coupling is easier to keep sufficiently low, since the designs are typically based on a single resonating mode, whereas in wideband antennas multiple modes need to be treated simultaneously.
In this paper, we introduce a novel wideband zeroclearance 8-element MIMO antenna design which is realized solely in the metal rim.Each antenna element consists of two properly designed slots and coupling in the MIMO system is minimized with a specific non-uniform arrangement of the elements.The proposed design simultaneously achieves the following three highly desirable properties: 1) a wide frequency band covering all 5G sub-6 GHz bands without any matching or decoupling structures, 2) a metal rim making the design suitable for smartphones with a metal frame, 3) a zero ground clearance enabling full edge-to-edge screen.
A systematic evolution and performance analysis of the proposed antenna design is presented.By adequately designing the shape and dimensions of the slots, the length of a single antenna element can be significantly reduced, about 50% compared to the initial design with straight slots, with improved wideband performance.The analysis of the current and distributions of a single antenna element in turn indicates that the wideband operation is based on the simultaneous excitation of multiple resonating and interacting modes of the slots.Another crucial property in the proposed design is that multipole modes can be excited with a single feed.
A prototype of the proposed 8-element MIMO antenna is manufactured and its operation is verified with measurements.The measurement results show competitive performance and the 3.4-6.1 GHz band with isolation better than 11.7 dB is achieved without any matching or decoupling structures.Finally, the hand effect is analyzed with simulations demonstrating that the hand does not have a significant effect on the −6 dB band of the proposed design.

II. SINGLE ANTENNA DESIGN
Typical wideband MIMO solutions include L-or T-shaped open slots placed both in the metal rim and the ground plane [15], [18], [19], [24].Wideband operation is obtained by exciting two or more distinct modes resonating at different but overlapping frequency bands.These modes obey either different current paths or distributions, or both.For example, by a properly placed feeding element, supplemented by a capacitor [15], or a tuning stub [19], two modes can be excited with a single feed.In [18] four modes are excited with two feeds.The design proposed in [16] includes T-and C-shaped slots placed solely in the rim.That sophisticated design supports four modes, but the S-parameter curve verifies that only two radiating modes are excited with a single feed.Furthermore, the solutions of [15], [18], [24], and [19] utilize lumped elements or matching structures to improve the performance of the design.
Our first goal is to design a single-port excited slot antenna placed entirely in the metal rim, having a compact size, and providing wideband operation without any lumped elements for input or aperture matching.The antenna design is performed with a simplified model for a smartphone.The total size of the model is 80 mm×160 mm×7 mm and a 0.76 mm thick Rogers RO4350B (ε r = 3.66 and tan δ = 0.0031) substrate is placed on top of the ground place and behind the longer sides of the rim.
We begin by presenting the single antenna element design in Section II-A, followed by a detailed analysis of the operation of the antenna in Section II-B.The initial design is the GND-slot antenna with the length of 32.5 mm including a slot on both the rim and the ground plane.This design is shown in Fig. 1(a).The dimensions, i.e., the lengths of the slots, and their perimeter (shortest current path around the slots), are defined so that the operation band of the design is roughly 3.4-6.0GHz.Finally, in Section III the design of an 8-element MIMO antenna with adequate isolation is presented.All antenna simulations in this paper are performed with CST Studio Suite 2022.In the first step, the slot on the ground plane (GNDslot) is moved to the rim (TI-slot) to obtain a zero-clearance design.As the results in Fig. 2 demonstrate, the GND and TI-slot antennas have almost identical −6 dB bandwidths.The next goal is to reduce the length of the antenna by bending the I-slot.This is analyzed with parameter a, shown in Fig. 1(c).Parameter a is varied from 0 to 9 mm so that another parameter b, also shown in Fig. 1(c), is defined as b = 9.65 mm−a.This guarantees that the perimeter of the closed blue slot is the same in all evolution steps.The results for the S-parameters of the evolution from the TI-slot to the TJ-slot, with respect to the parameter a, are shown in Fig. 3.These results illustrate that the TJ-slot, with a > 8.0 mm, significantly increases the −6 dB bandwidth compared to the TI and GND designs.In other words, bending the I-slot does not only reduce the total length of the antenna, from 32.5 mm (TI-slot) to 18.5 mm (TJ-slot), but also increases the bandwidth.

A. ANTENNA ELEMENT EVOLUTION
Next, the same bending process is applied to the T-slot.The final result is the FJ-slot antenna shown in Fig. 1(d).The achieved −6 dB band, 3.3-7.4GHz, has increased about 1.3 GHz compared to the initial GND-slot design.Moreover, the length of the antenna is remarkably reduced: the proposed design is roughly one-half of the original design, 16.5 mm (FJ-slot) vs. 32.5 mm (GND-slot).Although the length reduction obtained by the FJ design compared to the TJ design is only 2 mm, even a small size reduction is important because the benefit is multiplied in the high-order MIMO configuration directly reducing the coupling.This is analyzed in detail in Section III.
The key factor in the above evolution process is that the perimeters of both the open (yellow) and the closed (blue) slots are almost constant during the slot bending.The length of the slot perimeter is defined by the shortest possible current path and therefore the overall length of the antenna cannot be reduced infinitely by increasing the number of the bends in the slot.To shorten the antenna and to retain proper radiation properties, both the slot and the metal parts should be thick enough compared with the wavelength.The limiting dimension in the slot bending is the height of the rim (7 mm).By increasing the height of the rim, the design of a compact wideband antenna becomes easier.

B. WIDEBAND OPERATION WITH MERGING RESONATING MODES
The phenomena behind the slot bending are studied by analyzing both the surface currents, Fig. 4, and the electric fields, Fig. 5.The analysis is presented only for the TI and FJ-slot designs but the same principles can be also observed in all considered antenna evolution steps.
When the position of feeding port and the perimeter of both the open (yellow) and the closed (blue) slot are kept constant, while bending the slots, the same current and electric field modes will be excited as can be seen from Figs. 4 and 5.The first mode with the lowest resonance frequency, near 4 GHz, focuses on the open (yellow) slot.This mode has a single current maximum (near the feed) and a single null (at the slot opening).The second mode, resonating around 5.5 GHz, is focused on the open slot similarly to the first one except it has two maxima and two nulls, as shown in Fig. 4(b).The third mode, resonating at 6.5 GHz (TI-slot) or at 7 GHz 111890 VOLUME 11, 2023 Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.(FJ-slot), similarly to the second one, has two maxima and two nulls, but it is focused on the closed (blue) slot.
The performance of the FJ-slot antenna is next studied when the length of the J-slot (parameter x in the inset of Fig. 6) is increased from 0 mm to 9 mm.The last value, x = 9 mm, gives the proposed FJ design.The S-parameter results in Fig. 6 show three phenomena.Firstly, the F-slot with a very short J-slot has two separate resonances, near 3.5 GHz and 5.5 GHz.These resonances correspond to the current and field modes shown in Fig. 4(a) and (b), and Fig. 5(a) and (b).As the J-slot becomes longer, i.e., parameter x is increased, matching between these two separate resonances increases and their −6 dB bands merge, resulting wideband operation up to 5.8 GHz.Interestingly, this band is almost the same as the band of the TI-slot antenna shown in Fig. 1.Thirdly, as the parameter x is increased further, a third resonance due to the J-slot shifts downwards in frequency and appears on the frequency range of interest.
In conclusion, the analysis performed in this section illustrates that the ultra-wide 3.4-7.4GHz band of the proposed FJ design is due to the interaction of three resonating modes and their simultaneous excitation with a single feed.

III. MIMO ANTENNA DESIGN
The space between the MIMO antenna elements in a mobile phone is typically rather small.Especially this is the case for high-order MIMO antenna with several elements, where coupling between the elements becomes an issue.The simplest way to reduce coupling in the MIMO system is to increase the distance between the elements.But, due to the limited space in mobile phones, the distance between the antennas, and their feeding points, may not be increased  enough.Secondly, in the wideband designs handling coupling between the antenna elements in a wide frequency band is challenging.For single narrow-band antennas coupling is easier to keep sufficiently low, since the designs are typically based on a single resonating mode, whereas in wideband antennas multiple modes need to be treated simultaneously.Therefore in this section, we study how adequate coupling in wideband MIMO antenna can be achieved with a proper antenna element arrangement.The optimal antenna element arrangement, and the element spacing, are found by analysing the surface currents and identifying the locations of the current maxima.

A. MIMO ARRANGEMENT AND COUPLING ANALYSIS
Coupling in the MIMO system is studied by keeping the distance between two antenna elements fixed (15 mm) and by changing the antenna element arrangement.The target level for the coupling is below −10 dB which is widely used in mobile devices.Figure 7 shows coupling (the transmission coefficient S 21 ) between two identical antenna elements with three different arrangements, back-to-back, face-to-face, and back-to-face.Figure 7 shows that above 5.0 GHz the antenna element arrangement has a significant effect on coupling.To study this more closely, Fig. 8 shows the surface currents of the back-to-back and face-to-face arrangements at 5.8 GHz, and at 7.0 GHz, when only the antenna on the right is fed.At 5.8 GHz, Fig. 8(a), the currents are stronger on the F-slot than on the J-slot.Hence, the back-to-back arrangement, where the antennas are arranged so that the J-slots are against each other, gives the lowest coupling.At 7.0 GHz, Fig. 8(b), the situation is different and the strongest currents concentrate around the J-shaped slot.Consequently, above 6.3 GHz the face-to-face arrangement has the lowest coupling.
Below 5.0 GHz the antenna element arrangement seems to have only a minor effect on coupling.As can be seen from Fig. 7, at those frequencies the face-to-face arrangement gives the lowest coupling.Figure 7 shows also that with the face-to-face arrangement coupling is below −10 dB on the entire frequency band of interest.However, all antenna elements in an 8-element (four antennas per a long edge of the device) MIMO cannot be arranged into the face-to-face configuration.Therefore, only the two center antenna elements are placed in the face-to-face arrangement.The outermost antenna elements are in the back-to-face configuration, which is a trade-off between the other two, with a longer spacing between the elements.By slightly increasing the distance between the antenna elements guarantees that coupling is sufficiently low (below −10 dB) on the entire frequency band.In conclusion, the proposed design has adequately low coupling without any additional decoupling structures.This is an important feature, since the decoupling structures locates typically on the ground plane, which turn do not allow full screen devices.

B. PROTOTYPE DESIGN AND MANUFACTURING
To verify the performance of the antenna design obtained with simulations, a prototype shown in Fig. 10 is manufactured.The main body of the phone, i.e., the rim and the ground plane, is milled out of brass.The wall thickness of both the rim and the ground plane is 1 mm.The bottom of the brass frame is 2 mm thicker in the middle area to simplify the milling process.
The antennas are fabricated on a 0.76-mm thick Rogers RO4350B printed circuit board (PCB).The antenna PCB is galvanically connected to the brass frame with M2.5-sized nylon screws and nuts.Antenna dimensions for the prototype are re-tuned due to a different feeding structure.However, the basic operation principle is the same as in the simplified simulation model.
The antenna feeding is realized on a separate 0.76-mm thick Rogers RO4350B PCB with a 50 and 20-mm long microstripline, which is connect to the antenna with a via (diameter 0.4 mm).The length of the microstipline is chosen 111892 VOLUME 11, 2023 Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.to be 20 mm to minimize the effect of the measurement cable on the antennas.Similarly in the antenna PCB, the feeding PCB is pressed against the brass frame with M3sized nylon screws and nuts.The proper galvanic connection between the perpendicular antenna and the feeding PCBs is done by soldering the microstripline and the feeding point of the antenna together.The antennas are fed through SMA connectors that are screwed into the brass frame.

IV. PROTOTYPE SIMULATION AND MEASUREMENT RESULTS
S-parameters are measured with a two-port vector network analyzer (VNA).Figures 11 and 12 show the simulated and measured S-parameters.The results are in line showing similar overall shape of the S-parameters.The achieved −6-dB bandwidth is 3.4-6.1 GHz and coupling is below −11.6 dB.
The simplified antenna design simulated in Section III, with a different feeding structure, achieves a wider bandwidth than the measured one.The fabricated prototype has 20-mm long microstrip feeding lines and its band ends at 6.1 GHz.With shorter microstrip lines the achieved bandwidth would be wider but the effect of the measurement cable is minimized with longer lines.In a real mobile phone, this is not an issue since any measuring device is not needed and short microstrip lines can be used.
The simulated and measured total efficiencies are shown in Fig. 13.The achieved simulated total efficiency is 66.1-84.4% and the measured one is 57.6-84.9%.The simulation and measurement results agree well in the entire frequency band, except around 5 GHz where the measured total efficiency is about 5-10 percentage points lower.The small ripple visible in the efficiency results likely originates from the near-field measurement process, as it is not visible in the S-parameter results.
Figure 14 shows the calculated ergodic capacity for both the measured and simulated results.The ergodic capacity is calculated as in [26] with a signal-to-noise ratio (SNR) of 20 dB and a Rayleigh fading channel with 10 4 realizations.For comparison, the theoretical ergodic capacity of an 8-element MIMO is plotted with a black dashed line.The envelope correlation coefficient (ECC) is computed from both the simulated and measured far-field radiation pattern as presented in [27].The ECC is very small, below 0.07, in the entire frequency band, as shown in Fig. 15.
The hand effect on the MIMO antenna is simulated by using CST's CTIA hand model.Figure 16 visualizes the simulation model where the mobile device is held in the hand.The corresponding S-parameters are shown in Fig. 17.These results indicate that the hand does not have any significant impact on the −6-dB bandwidth, and only the antenna number 6, which is covered by the ring finger, has a slightly narrower band.Total efficiency of the antenna with the hand varies between 10% and 80% depending how much an antenna element is covered by the hand.We may conclude that the overall performance of the antenna is very good also with the hand.the measured results only.We observe that slightly smaller antennas than the proposed one have been introduced, but these designs utilize either matching networks or they do not have zero ground clearance or metal rim.Similarly, antennas with better isolation can be achieved, but with additional decoupling structures.

V. COMPARISON WITH PUBLISHED RESULTS
From Table 1 we observe that the proposed design has the following three important properties: a zero ground clearance allowing a full edge-to-edge screen, a metal rim enabling metal-frame smartphones, and a wide frequency band covering all 5G sub-6 GHz frequency bands without any matching or decoupling structures.All these features are highly desirable in modern mobile phones.Additionally, we note that the proposed antenna design shows competitive bandwidth, isolation, and total efficiency.

VI. CONCLUSION
This paper presents a novel wideband zero ground clearance 8-element mobile MIMO antenna.The antenna design is simple, two slots in the rim, and does not include any decoupling structures or matching elements.Due to zero ground clearance, and compact size, the proposed antenna is suitable for modern metal rim 5G smartphones with full edge-to-edge screen.Performance analysis of the design illustrates that the wideband operation of the antenna is achieved by simultaneous excitation of three current modes with overlapping resonance bands.
A prototype of the antenna is manufactured to verify the simulation The 8-element MIMO antenna achieves wide −6 dB-band 3.4-6.1 GHz, good total efficiency 57.6-84.9%,and high isolation 11.7 dB. the proposed antenna achieves about 87.2-94.8% of the theoretical 8-element ergodic capacity.The simulation results show that with an alternative implementation of the feeding structure, the bandwidth of the proposed antenna design could be extended up to 7.3 GHz.

FIGURE 1 .
FIGURE 1. Antenna design evolution: (a) Initial design, GND-slot, intermediate designs (b) TI-slot, and (c) TJ-slot, (d) final design, FJ-slot.Open slots are marked with yellow, closed ones with blue, and the red rectangle denotes the feeding port.Figures are not in scale and only part of the rim and ground plane is shown.In (c) parameters a and b (a + b = 9.65 mm) are used to obtain a transition from the TI design to the TJ one.

Figure 1
Figure1visualizes the single antenna element design evolution from the initial design, Fig.1(a), to the final one, Fig.1(d).The names of the antennas describe the shape or location of the slots.The functional parts of the antennas are colour coded.The feeding port (red) of each antenna design divides the structure into two slots: an open slot on the left (yellow) and a closed one on the right (blue).In each

FIGURE 2 .
FIGURE 2. S-parameters with different slot designs of Fig. 1.The TJ-slot of Fig. 1(c) is defined with a = 9.0 mm and b = 0.65 mm.

FIGURE 3 .
FIGURE 3. S-parameters for the antenna design evolution from the TI-slot (Fig.1(b) ) to the FJ-slot (Fig.1(d)).The legend specifies the parameter a defining evolution from the TI-slot to the TJ-slot.

FIGURE 4 .
FIGURE 4. Surface currents of the TI (left) and FJ (right) slot designs at three resonance frequencies.(a) 4.0 GHz, (b) 5.5 GHz, and (c) 6.5 GHz (TI) and 7 GHz (FJ).The strongest current paths are highlighted with pink arrows.The dimensions are not in scale.

FIGURE 5 .
FIGURE 5. Electric fields of the TI-(left) and FJ-slots (right) at three resonance frequencies.(a) 4.0 GHz, (b) 5.5 GHz, and (c) 6.5 GHz (TI) and 7 GHz (FJ).The dimensions of the antennas are not in scale.

FIGURE 6 .
FIGURE 6. S-parameters of the FJ-slot antenna with different lengths of the J-slot (parameter x in the inset).The proposed FJ-slot design is obtained with x = 9 mm.

FIGURE 7 .
FIGURE 7. Coupling of two identical FJ-slot antennas with three different antenna element arrangements.(a) Back-to-back, (b) face-to-face, and (c) back-to-face.The distance between the antennas is the same, 15 mm, in all cases.

FIGURE 8 .
FIGURE 8. Surface currents of the back-to-back (up) and face-to-face (down) arrangements of two identical antenna elements at (a) 5.8 GHz and at (b) 7.0 GHz.The excitation port is on the antenna on the right.Only part of the rim and ground plane is shown.

FIGURE 9 .
FIGURE 9. S-parameters of the 8-element MIMO antenna.All S-parameters are not shown due to the symmetry of the design.Ports are numbered counterclockwise so that ports 5-8 are opposite to ports 4-1.

FIGURE 10 .
FIGURE 10.Manufactured prototype.The feeding point via connects the antenna to the microstripline.PCBs are connected to the brass frame with (white) nylon screws.The dimensions are in mm.

Figure 9
Figure 9 shows the final antenna element arrangement in the proposed 8-element MIMO design.The S-parameters of this antenna are shown in Fig. 9.The obtained −6 dB bandwidth is very wide, 3.3-7.3GHz, and coupling is adequate, below −11.4 dB.

FIGURE 11 .
FIGURE 11.Matching of the proposed 8-element MIMO antenna prototype.(a) Simulated, (b) measured S-parameters.All simulated results are not plotted due to a symmetrical structure.

FIGURE 14 .
FIGURE 14.Ergodic capacity of the proposed antenna prototype.The theoretical upper limit for an 8-element MIMO is plotted with a black dashed line.

FIGURE 15 .
FIGURE 15.ECC calculated from the measured far-field pattern.

FIGURE 17 .
FIGURE 17. Simulated S-parameters with a hand.The simulation setup is shown in Fig. 16.(a) Matching.(b) Coupling.

TABLE 1 .
Comparison of measured wideband 8-element sub-6 GHz MIMO mobile phone antennas.

Table 1
presents a comparison of the proposed design (Prop.)with previously introduced wideband sub-6 GHz MIMO mobile antennas.The data shown in the table is based on